Infection by enveloped viruses is initiated by binding of the viral envelope glycoprotein(s) to specfic receptor molecules on the target cell, followed by fusion between the viral and cellular membranes. We have been studying various aspects of viral envelope glycoprotein (env)/receptor interactions: Human immunodeficiency virus (HIV) env glycoprotein/CD4 interactions. Using a recently developed reporter gene assay system to measure fusion between env-expressing and CD4-expressing cells, we have explored several aspects of HIV fusion: a) Our previous results indicated that fusion requires the presence of a human-specific accessory factor(s) on the CD4-expressing cell. We initiated molecular genetic approaches to identify this factor. Preliminary results indicate that murine cells expressing human CD4 can by rendered fusion-competent by microinjection of mRNA from a human cell (HeLa), or by transfection with a HeLa cDNA library. Isolation of the active cDNA is in progress. b) We previously demonstrated that the tropism of different HIV-1 isolates for infection of CD4-expressing T-cell lines vs. primary macrophages is associated with the intrinsic fusion specificities of the corresponding envs. Recent experiments indicate that treatment of monocyte cell line with differentiating agents renders them susceptible to fusion by envs from macrophage tropic isolates, in parallel with acquisition of infection for such isolates. Experiments with transient cell hybrids reveal that T-cell line vs. macrophage tropism is associated with cell type-specific accessory fusion factors. Experiments are in progress to identify the accesory factor(s) associated with entry of macrophage-tropic isolates. c) We have optimized conditions for the reporter gene assay to measure fusion inhibition by antibodies and pharmacoplogical agents.